Dimmer control leakage pull down using main power device in flyback converter

ABSTRACT

A flyback controller-may include a dimmer input configured to receive a chopped and rectified AC voltage. Each cycle of the signal may have an off period which is substantially attenuated but not always zero due to leakage of a dimmer control from which the chopped AC voltage originates, and an on period which substantially tracks the AC voltage. The ratio of the off period to the on period may be dependent upon a setting of the dimmer control. The flyback controller may include a control circuit configured to generate a switching signal based on the signal from the dimmer input. The switching signal may controllably oscillate between its on and off states during the on periods of the chopped and rectified AC voltage so as to controllably regulate current that is delivered by a secondary winding of a transformer in a flyback converter. The switching signal may be in the on state during the off periods of the chopped and rectified AC voltage, thereby preventing a voltage build up from the dimmer control leakage.

BACKGROUND

1. Technical Field

This disclosure relates to light emitting diodes (LEDs), dimmercontrols, flyback controllers, and power factor correction.

2. Description of Related Art

Cold cathode fluorescent lamps have long-since been used in offices andhave become popular in the home. Compared to incandescent lamps, theirlumens per watt may be very high, saving energy. However, they mayrequire a high voltage AC inverter and may contain toxic mercury.

Light-emitting diodes (LEDs) are also now capable of providing highlight output per watt, comparable to cold cathode fluorescent lamps.Unlike cold cathode fluorescent lamps, moreover, they may not requirehigh voltage and do not usually contain mercury.

Driving LEDs from the 110 volt alternating line current that istypically available, however, may be challenging. Unlike incandescentlamps, for example, the intensity of an LED may be proportional to thecurrent which is delivered through it, not the amount of voltage that isapplied across it. Thus, circuitry may be needed to convert the linevoltage to a constant current. It may also be desirable to configurethis circuitry so that it may drive the LED from the output of aconventional dimmer control, such as one that uses a triac.

One approach has been to convert the output of the dimmer control to aconstant current using a flyback converter. Some dimmer controls,however, leak current during periods when their triacs are not firing,i.e., during off periods of their chopped AC voltage output. Thisleakage may cause a voltage build-up in the flyback controller, whichmay cause noise, flickering, and/or other undesirable results.

Dedicated hold-off circuits with low current limits have been added inan attempt to address this problem. These circuits may prevent thesupply voltage from increasing to a point that might otherwise cause theflyback converter to be energized. However, these dedicated hold-offcircuits may require an additional high voltage active device, such as abipolar transistor. This may add costs, complexity, and size.

SUMMARY

A flyback controller may generate a switching signal that causes aswitching system in a flyback converter to deliver current to a primarywinding of a transformer in the flyback converter while the switchingsignal is in an on state and to stop delivering that current while theswitching signal is in an off state. The flyback controller may includea dimmer input configured to receive a dimmer signal reflective of achopped and rectified AC voltage. Each cycle of the dimmer signal mayhave an off period which is substantially attenuated but not always zerodue to leakage of a dimmer control from which the chopped AC voltageoriginates. Each cycle of the dimmer signal may have an on period whichsubstantially tracks the AC voltage. The ratio of the off period to theon period may be dependent upon a setting of the dimmer control. Theflyback controller may include a control circuit configured to generatethe switching signal based on the dimmer signal from the dimmer input.The switching signal may controllably oscillate between its on and offstates during the on periods of the chopped and rectified AC voltage soas to controllably regulate the current that is delivered by a secondarywinding of the transformer. The switching signal may be in the on stateduring the off periods of the chopped and rectified AC voltage, therebypreventing a voltage build up from the dimmer control leakage.

A powered LED circuit may include a flyback converter. The flybackconverter may include a rectification system configured to produce thechopped and rectified AC voltage by rectifying a chopped AC voltage froma dimmer control. The flyback converter may include a transformer havinga primary winding and a secondary winding, a switching system configuredto controllably deliver current into the primary winding of thetransformer, and a flyback controller of the type described. The poweredLED circuit may include one or more LEDs configured to receive currentthat is delivered by the secondary winding of the transformer.

These, as well as other components, steps, features, objects, benefits,and advantages, will now become clear from a review of the followingDetailed Description of Illustrative Embodiments, the accompanyingdrawings, and the claims.

BRIEF DESCRIPTION OF DRAWINGS

The drawings disclose illustrative embodiments. They do not set forthall embodiments. Other embodiments may be used in addition or instead.Details that may be apparent or unnecessary may be omitted to save spaceor for more effective illustration. Conversely, some embodiments may bepracticed without all of the details that are disclosed. When the samenumeral appears in different drawings, it is intended to refer to thesame or like components or steps.

FIG. 1 is a block diagram of an LED circuit powered by a dimmer controland a flyback converter.

FIG. 2 illustrates a chopped AC output from a dimmer control.

FIG. 3 illustrates a portion of a flyback converter including a flybackcontroller that includes a output current monitoring circuit.

FIG. 4 illustrates selected waveforms that may be found during operationof a flyback converter containing the circuitry illustrated in FIG. 3.

FIG. 5 illustrate a portion of the flyback converter illustrated in FIG.3 configured to adjust the desired peak input current to effectuatepower factor correction.

FIG. 6 illustrates power factor corrections that the circuit illustratedin FIG. 5 may provide as a function of the phase angle of the chopped ACvoltage.

FIG. 7 illustrates power factor corrections that the circuit illustratedin FIG. 5 may provide as a function of the output voltage of the flybackconverter.

FIG. 8 illustrates the portion of the flyback converter illustrated inFIG. 5 configured to adjust the desired average peak input current toeffectuate power factor correction.

FIG. 9 illustrates a current ripple reduction circuit.

FIG. 10 illustrates part of a flyback controller that may be used in aflyback converter driven by a dimmer control to enhance the perceivedlinearity between changes in settings of the dimmer control andcorresponding changes in the intensity of light from one or more LEDsdriven by the flyback converter.

FIG. 11 is a graph of output current as a function of dimmer controlsettings for various flyback converter designs.

FIG. 12 illustrates a flyback controller configured to prevent voltagebuildup in a flyback converter that is being driven by a dimmer controldue to leakage in the dimmer control.

FIG. 13 illustrates waveforms that may be present in the flybackcontroller illustrated in FIG. 12.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Illustrative embodiments are now discussed. Other embodiments may beused in addition or instead. Details that may be apparent or unnecessarymay be omitted to save space or for a more effective presentation.Conversely, some embodiments may be practiced without all of the detailsthat are disclosed.

FIG. 1 is a block diagram of an LED circuit powered by a dimmer controland a flyback converter. As illustrated in FIG. 1, LEDs 101 may bepowered by a power supply 103 that receives AC power.

The number of the LEDs 101 may vary. For example, there may be two,three, five, ten, twenty-five, or a different number. Although referredto in the plural, there may be only a single LED.

The LEDs 101 may be connected in series or in parallel or in acombination of series and parallel. The particular configuration maydepend upon the amount of current and voltage which is available todrive the LEDs 101.

The LEDs 101 may be of any type. For example, they may operate at anyvoltage, at any current, and/or produce any color or combination ofcolors. The LEDs 101 may all be of the same type of may be of differenttypes.

The power supply 103 may be of any type. For example, the power supply103 may include a dimmer control 105 and a flyback converter 107.

The dimmer control 105 may be of any type. For example, the dimmercontrol may include a triac 109 configured with associated circuitry toprovide a chopped AC voltage output based on a setting of the dimmercontrol, such as the rotational position of a knob, the longitudinalposition of a slider, and/or the amount of time by which a touch platehas been touched.

The triac may be configured to function as a switch. When open, theremay be essentially no output from the triac, except for leakage. Whenclosed, the full magnitude of the AC voltage may be delivered to theoutput.

The switching of the triac from off to on may be governed by theinjection of a signal into a gate of the triac. The circuitry associatedwith the triac may cause the signal to be injected into the gate at apoint in time that corresponds to a phase angle of the alternatingcurrent that corresponds to a setting of the dimmer control.

FIG. 2 illustrates a chopped AC output from a dimmer control. Asillustrated in FIG. 2, a chopped AC output 201 may be off during an offperiod 203. A triac may be turned on by a signal at its gate at a phaseangle that corresponds to a setting of the dimmer control, such as at 60degrees as illustrated in FIG. 2. The chopped AC output from the dimmercontrol may then remain on during an on period 205 until the magnitudeof the AC voltage reaches approximately zero at a phase angle of 180degrees. Once the current though the triac 109 reaches approximatelyzero, intrinsic characteristics of the triac 109 may cause the triac 109to shut off. This may prevent any further output from the dimmer control105 until the triac is again fired by another signal to its gate

The gate of the triac 109 may again be energized, again at a phase anglethat is set by the associated circuitry in the dimmer control 105 basedon a setting of the dimmer control. This may cause the cycle illustratedin FIG. 2 to repeat. However, it may do so in connection with theremaining negative half of the AC cycle (which is not illustrated inFIG. 2). Thus, the next cycle may be a negative cycle, but may otherwisebe identical to the one illustrated in FIG. 2.

A devices other than the triac 109 may be used in addition or instead.For example, two SCR's may be used instead. Even a single SCR may beused, but this may result in only the positive or negative portion ofthe AC voltage being outputted from the dimmer control 105.

Returning to FIG. 1, the flyback converter 107 may be of any type. Theflyback converter 107 may include a rectification system 111, an outputfilter 113, a flyback controller 115, a switching system 117, atransformer 119, a rectification system 121, and/or an output filter123.

The rectification system 111 may be of any type. For example, it mayinclude a full wave bridge rectifier. Such a full wave bridge rectifiermay be configured to convert the positive and negative chopped portionsof the AC voltage that are delivered by the dimmer control 105 into allpositive chopped portions or into all negative chopped portions, i.e.,into a chopped and rectified AC voltage. A half wave bridge rectifiermay be used instead, in which case either the positive or negativechopped portions of the output from the dimmer control 105 may be lost.

The output filter 113 may be of any type. The output filter 113 may beconfigured to filter the chopped and rectified AC voltage from therectification system 111. For example, the output filter 113 may be alow pass filter. To minimize costs, size, and for other reasons, theamount of filtering provided by the output filter 113 may be minimal. Ifa low pass filter is used, for example, the low pass filter may have acutoff frequency that is substantially above the ripple frequency of thechopped and rectified AC voltage from the rectification system 111. Forexample, it may be sufficient to filter out high frequency noise in thechopped and rectified AC voltage, but not to sustain the output of theoutput filter 113 during substantial portions of the off periods of thechopped and rectified AC voltage.

The output filter 113 may include a capacitance. The capacitance may beof any value. It may be less than one microfared, such as approximately0.5 microfared or 0.1 microfared.

The output from the output filter 113 may be delivered to the flybackcontroller 115 and to the switching system 117.

The flyback controller 115 may be of any type. The flyback controller115 may be configured to generate a switching signal for controllingdelivery of current into a primary winding of the transformer 119. Theflyback controller 115 may be configured to generate the switchingsignal in a manner that causes a constant average output current to bedelivered to the LEDs 101 that is a function of the average value of thechopped and rectified AC voltage.

To effectuate this control, the flyback controller 115 may deliver aswitching signal to the switching system 117. The switching system 117may be configured to connect the primary winding of the transformer 119to the chopped and rectified AC voltage from the output filter 113 inconformance with the switching signal received from the flybackcontroller 115.

The switching system 117 may be of any type. For example, it may includeone or more electronic switches, such as one or more FETS, MOSFETS,IGBTs, and/or BJTs. The switching system 117 may include one or morelogic devices that may be used to cause the electronic switches toswitch the primary winding of the transformer 119 between the outputfrom the output filter 113 and ground based on the switching signal fromthe flyback controller 115.

The transformer 119 may be of any type. As indicated, it may have aprimary winding which is connected to the output of the output filter113 through the switching system 117 based on the switching signal. Thetransformer 119 may include a secondary winding which may be connectedto the rectification system 121. The transformer 119 may include one ormore additional primary and/or secondary windings, which may be used forother purposes. The turns ratio(s) and other characteristics of thetransformer 119 may vary.

The rectification system may be configured to rectify the output fromthe secondary winding of the transformer 119. For example, therectification system 121 may include one or more diodes. Half waverectification may be used.

The output of the rectification system 121 may be connected to theoutput filter 123. The output filter may be configured to filter theoutput from the rectification system 121. The output filter may includea capacitance. The capacitance may or may not be sufficient tosubstantially sustain the output from the rectification system 121through off periods of the chopped and rectified AC voltage.

The flyback converter 107 may be configured to deliver an output fromthe output filter 123 to the LEDs 101 that is DC isolated from thechopped AC voltage from the dimmer control 105. The flyback converter107 may be configured to do so without using any opto-isolator, such asan opto-isolator that provides feedback indicative of the output currentfrom the secondary winding in the transformer 119.

FIG. 3 illustrates a portion of a flyback converter including a flybackcontroller that includes an output current monitoring circuit. Thecircuitry illustrated in FIG. 3 may be used in connection with thedimmer-powered LED circuit illustrated in FIG. 1, in other types ofdimmer-powered LED circuits, or in other types of circuits, such as ingeneral purpose flyback converters that are configured to generate aconstant-current output. Similarly, the dimmer-powered LED circuitillustrated in FIG. 1 may be implemented with circuitry other than isillustrated in FIG. 3.

As illustrated in FIG. 3, a transformer 301 may have a primary winding303 and a secondary winding 305. The transformer 301 may correspond tothe transformer 119 illustrated in FIG. 1. The transformer 301 may be ofany type. It may have one or more additional primary and/or secondarywindings, and it may have any turns ratio.

The primary winding 303 of the transformer 301 may be connected to asource of power. Any type of power may be used. For example, the sourceof power may be a DC source, a full wave rectified AC source, a halfwave rectified AC source, or a chopped and rectified source of powerfrom a dimmer control, such as the output from the output filter 113illustrated in FIG. 1.

The secondary winding 305 of the transformer 301 may be rectified by adiode 307. The diode 307 may correspond to the rectification system 121illustrated in FIG. 1. The output from the diode 307 may be filtered bya capacitor 309. The capacitor 309 may correspond to the output filter123 illustrated in FIG. 1. The capacitor 309 may or may not besufficient to substantially sustain the output from the rectificationsystem 121 through off periods of the chopped and rectified AC voltage.

One or more LEDs may be connected to the output of the capacitor 309,such as the LEDs 311, 313, and 315. The LEDs 311, 313, and 315 maycorrespond to the LEDs 101 illustrated in FIG. 1 and may be any of thetypes discussed above in connection with FIG. 1. Although illustrated asbeing connected in series, the LEDs 311, 313, and 315 may be connectedin parallel and/or in a combination of series and parallel. Anydifferent number of LEDs may be used instead.

An FET 317 may be used to controllably connect the other side of theprimary winding 303 to ground through a sense resistor 319. The FET 317may correspond to the switching system 117 illustrated in FIG. 1. Othertypes of switching systems may be used in addition or instead. Theswitching system may instead be inserted in series with the other sideof the primary winding 303 of the transformer 301.

The circuit illustrated in FIG. 3 may be configured to maintain theaverage output current in the secondary winding 305 substantiallyconstant, as will become more clear from the discussion below. Toaccomplish this, the circuitry may monitor the current in the secondarywinding.

That current may be monitored by measuring the voltage on the primarywinding 303 during periods of time when the secondary winding 305 isconducting current. A different approach, however, is taken in FIG. 3.The theory underlying this different approach is now presented.

In a flyback converter, such as is partially illustrated in FIG. 3, theprimary winding of a transformer, such as the primary winding of thetransformer 301, may be connected through a switching system, such asthe FET 317, to a source of current. This may cause current to steadilybuild in the primary winding 303 based on the amount of voltage which isapplied and the amount of inductance in the primary winding. Acorresponding voltage may be simultaneously generated on a secondarywinding of the transformer, such as the secondary winding 305. However,no current may yet flow in the secondary winding because a half waverectification system that may be attached to the secondary winding, suchas the diode 307, may be reversed biased.

The current in the primary winding may continue to grow until such timeas it reaches a desired peak value. At this point, the switching systemmay be turned off. The may cause the current through the primary windingto cease.

The magnetic field that was built up in the transformer due to thecurrent in the primary winding may now begin to transfer to thesecondary winding. This may cause the output voltage on the secondarywinding to change polarity, causing the half wave switching system, suchas the diode 307, to be forwarded biased. In turn, this may causecurrent to flow in the secondary winding.

The current in the secondary winding may begin at a peak value anddecrease to zero in approximately a linear fashion. Once it reacheszero, the switching system in the primary may again be turned on.Current may then again build in the primary winding. This entire processmay repeat.

This delivery current in the primary winding followed by current flowingin the secondary winding of the transformer may repeat at a very fastfrequency. The frequency may be greater than 100 KHz, such as at about200 KHz.

As indicated above, current may not flow in the secondary winding whileit is flowing in the primary winding. The relative amount of time duringwhich current flows in the secondary winding versus the amount of timeduring which current does not flow in the secondary winding may bereferred to as the duty cycle of the current in the secondary winding.

The average amount of current which flows in the secondary winding maybe proportional to the product of the peak value of the current whichinitially flows in the secondary winding times the duty cycle of thatcurrent. As the peak value increases, for example, the average amount ofcurrent may also increase, even if the duty cycle is not altered.Similarly, the average value of the current in the secondary winding mayincrease if the duty cycle increases, even if the peak value remains thesame.

The peak value of the current which initially flows in the secondarywinding may be proportional to the peak value of the current which isreached in the primary winding before the current in the primary windingis shut off by the switching system. Thus, the average value of thecurrent which flows in the secondary winding may be proportional to thepeak value of the current that is reached in the primary winding timesthe duty cycle of the current in the secondary winding.

An output current monitoring circuit may therefore be configured togenerate a signal representative of the average output current in thesecondary winding 305 based on the peak input current in the primarywinding 303 and the duty cycle of the current in the secondary winding305. Any circuitry may be used to measure these quantities and generatethis signal. As illustrated in FIG. 3, for example, the output currentmonitoring circuit may include the sense resistor 319, a peak inputcurrent sensing circuit 321, a pulse width modulator 323, and a low passfilter formed by a resistor 325 and a capacitor 327.

The sense resistor 319 may produce an input current signal 330 that hasa voltage that is representative of the current in the primary winding303 of the transformer 301. The sense resistor 319 may have a relativelylow resistance so as to not waste power. The voltage produced by thesense resistor 319 may be processed by the peak input current sensingcircuit 321. The peak input current sensing circuit 321 may beconfigured to generate an output which represents the peak value of thecurrent in the primary winding 303. To accomplish this, the peak inputcurrent sensing circuit 321 may include a sample and hold circuit. Thesample and hold circuit may be configured to sample the output from thesense resistor 319 while current is flowing in the primary winding 303and to hold the value of the current that is flowing immediately beforethe FET 317 is turned off. This value may be the peak value of thecurrent in the primary winding 303 due to the fact that the current maysteadily rise until the FET 317 is turned off.

A duty cycle signal 329 may be indicative of the duty cycle of currentin the secondary winding 305. The duty cycle signal 329 may be derivedfrom a memory, such as a D memory 331. The operation of the D memory 331will be described below.

The pulse width modulator may be configured to generate an output thatrepresents the peak input current from the peak input current sensingcircuit 321 multiplied by the duty cycle signal 329, thus creating apulse-width modulated version of the peak input current signal. The lowpass filter formed by the resistor 325 and the capacitor 327 may beconfigured to extract the average value of the pulse-width modulatedpeak input current, thus creating an average output current signal 333.The average output current signal 333 may therefore represent theaverage output current in the secondary winding 305 because, asexplained above, the average output current in the secondary winding 305may be proportional to the average value of the peak input current inthe primary winding 303 multiplied by the duty cycle of the outputcurrent in the secondary winding 305.

The low pass filter that is formed by the resistor 325 and the capacitor327 may have a cut-off frequency that is at least five times lower thanthe frequency of the chopped and rectified AC voltage, such asapproximately ten times lower. When the frequency of the AC voltage is60 hertz, for example, the frequency of the chopped and rectified ACvoltage may be 120 hertz. In this example, the cut-off frequency of thelow pass filter formed by the resistor 325 and the capacitor 327 maytherefore be approximately 12 hertz. The net effect of this low cut-offfrequency may be to produce the average output current signal 333 thataverages the output current in the secondary winding 305 over severalcycles of the chopped and rectified AC voltage.

An amplifier 335 may be configured in connection with the capacitor 327and the resistor 325 so as to form an integrator which integrates thedifference between a desired average output current signal 337 and theaverage output current signal 333. The output of the amplifier 335 maybe treated in the circuit as a desired peak input current signal 339,i.e., a signal representing the amount peak current in the primarywinding 303 that is needed to provide the desired average output currentin the secondary winding 305.

The state of the FET 317 may be controlled by the D memory 331. When theD memory 331 is set by a signal to its set S input, the Q output of theD memory output may go high. When set, this may cause the FET 317 toturn on which, in turn, may begin delivery of current into the primarywinding 303 of the transformer 301.

When a signal is delivered to the reset R input of the D memory, the Qoutput of the D memory may go low. When reset, this may cause the FET317 to turn off which, in turn, may stop delivery of current into theprimary winding 303 of the transformer 301.

The Q output of the D memory may represent an output that iscomplimentary to the Q output.

A boundary detect circuit 341 may be used to set the D memory 331 Theboundary detect circuit 341 may be configured to initiate current in theprimary winding 303 of the transformer 301 in accordance with any one ofseveral different types of timing schemes. For example, the boundarydetect circuit 341 may be configured to initiate current in the primarywinding 303 at the moment current in the secondary winding 305 reacheszero. The boundary detect circuit 341 may be configured to detect whencurrent in the secondary winding 305 ceases by monitoring the voltageacross the primary winding 303 while current is flowing in the secondarywinding 305.

A comparator 343 may be configured to output a signal which resets the Dmemory 331 and thus turns off the FET 317 at such time as the inputcurrent signal 330 reaches the level of the desired peak input currentsignal 339.

When the average output current signal 333 is less than the desiredaverage output current signal 337, the circuitry configuration that hasbeen discussed may therefore cause the desired peak input current signal339 to grow until such time as the average output current signal 333reaches the level of the desired average output current signal 337.Conversely, when the average output current signal 333 is more than thedesired average output current signal 337, the circuitry configurationthat has been discussed may cause the desired peak input current signal339 to get smaller until such time as the average output current signal333 gets back down to the level of the desired average output currentsignal 337.

The overall effect of the circuitry which has just been described maytherefore be to cause a constant average current to be delivered by thesecondary winding 305 that corresponds to the desired average outputcurrent signal 337. The circuitry may do so while the output of theflyback converter is electrically isolated from the AC voltage, allwithout using any opto-isolator, such as an opto-isolator that isconfigured to provide feedback indicative of the output current from thesecondary winding 305.

As indicated above, the chopped and rectified AC voltage from the outputfilter 111 may be used as a source of power to the primary winding 303.In this configuration, the boundary detect circuit 341 may be configurednot to set the D memory 331 during the off periods of the chopped andrectified AC voltage. Correspondingly, the integrator that is formed bythe amplifier 335, the resistor 325 and the capacitor 327 may bedisabled during these off periods, so as not to allow the value of theintegration to be changed by these off periods. In other words, thecircuit illustrated in FIG. 3 may be configured to cause the averagevalue of the output current in the secondary winding 305 to match thevalue represented by the desired average output current signal 337during the on periods of the chopped and rectified AC voltage, but notduring its off periods.

Separate power supply circuitry may be provided to generate a constantsource of DC power from the chopped and rectified AC voltage, regardlessof the chopped nature of this voltage. The output of this separate powersupply circuitry may be used to power the flyback controller, includingthe circuitry illustrated in FIG. 3, during off periods of the choppedand rectified AC voltage, as well as during its on periods.

FIG. 4 illustrates selected waveforms that may be found during operationof a flyback converter containing circuitry of the type illustrated inFIG. 3. As illustrated in FIG. 4, input current 401 may begin to riseeach time after the FET 317 is turned on. It may continue to rise untilit reaches the desired peak input current 403. Once the input current401 reaches the desired peak input current 403, the comparator 343 maysend a signal to the reset R input to the D memory 331, causing the FET317 to turn off.

At this point, current through the secondary winding 305 may begin toflow. The duty cycle of the current which flows in the secondary winding305 may be reflected at the Q output of the D memory 331. The pulsewidth modulator 323 may multiply the peak input current signal from thepeak input current sensing circuit 321 by the duty cycle signal 329,thus generating the pulse-width modulated peak input current signal 405.The average value of the pulse-width modulated peak input current signal405 may then be extracted by the low pass filter formed by the resistor325 and the capacitor 327, thus generating the average output currentsignal 333. If the average output current signal 333 does not match thedesired average output current signal 337, the integrator formed by theamplifier 335 and the capacitor 327 may continue to adjust the desiredpeak input current signal 339 until it does.

The circuitry which is illustrated in FIG. 3 may cause the current whichis drawn from the AC voltage to have a wave shape which is substantiallydifferent from the AC voltage. For example, while the AC voltage isfalling in value, such as when the phase angle of the AC voltage goesfrom 90 to 180 degrees (see FIG. 2), the circuit in FIG. 3 may cause theaverage current which is drawn by the flyback converter to remainsubstantially constant. This may result in a low power factor, such asbetween 0.6 and 0.7. Such a low power factor may require the utilitywhich supplies the line voltage to provide more current than is actuallyneeded. It may also cause problems with electromagnetic interference dueto sharp current spikes.

FIG. 5 illustrates a portion of the flyback converter illustrated inFIG. 3 configured to adjust the desired peak input current to effectuatepower factor correction. As may be apparent, the circuit illustrated inFIG. 5 is the same as the circuit illustrated in FIG. 3, except that amultiplier 501 has been inserted in the output of the amplifier 335, avoltage divider network consisting of resistors 503 and 505 has beenadded, and a chopped and rectified AC voltage input 507 has been added.

The circuitry modification may cause the output of the integrator formedby the amplifier 335, the resistor 325, and the capacitor 327, to bemultiplied by a signal representative of the chopped and rectified ACvoltage. This may cause the desired peak input current signal 339 totrack the instantaneous value of the chopped and rectified AC voltage.Thus, when the instantaneous value of the chopped and rectified ACvoltage increases or decrease, the value of the desired peak inputcurrent signal 339 may increase and decrease along with it. This maycause the wave shape of the average current which is drawn from thechopped and rectified AC voltage, such as from the output of the outputfilter 113, to more closely match the chopped and rectified AC voltage,thus increasing the power factor of the circuit. At the same time, thefeedback loop which remains in FIG. 5 and has been discussed above inconnection with FIG. 3, may still ensure that the average output currentmatches the desired average output current signal 337 during each onperiod of the chopped and rectified AC voltage.

FIG. 6 illustrates power factor corrections that the circuit illustratedin FIG. 5 may provide as a function of the phase angle of the chopped ACvoltage. As illustrated in FIG. 6, the input current 601 drawn by theflyback converter may closely track the input voltage 603 over the fullrange of phase angles to which the dimmer control may be set.

The power factor of the circuit illustrated in FIG. 5 may vary dependingupon the output voltage of the flyback converter. The graphs illustratedin FIG. 6 represent a relationship between input current and inputvoltage for an output voltage of approximately 50 volts. When the outputis at this voltage level, the power factor may be at least 0.8, at least0.9, at least 0.95, or at least 0.98 at each of the possible dimmerphase angles.

FIG. 7 illustrates power factor corrections that the circuit illustratedin FIG. 5 may provide as a function of the output voltage of the flybackconverter. As can be seen from FIG. 7, the power factor may remain veryhigh over a wide range of output voltages.

The circuitry in FIG. 5 seeks to provide power factor correction bycausing the desired peak input current to track changes in the inputvoltage. However, the average input current may not be directlyproportional to the desired peak input current. The average inputcurrent may also be a function of the duty cycle of the input current tothe primary winding 303, which may change as function of changes in theinput voltage. Thus, more power factor correction may be achieve bycausing the desired average input current to the primary winding 303 totrack changes in the input voltage, instead of the desired peak inputcurrent.

FIG. 8 illustrates the portion of the flyback converter illustrated inFIG. 5 configured to adjust the desired average peak input current toeffectuate power factor correction. As may be apparent, the circuitillustrated in FIG. 8 is the same as the circuit illustrated in FIG. 6,except that a second integrator has been added consisting of anamplifier 801, a capacitor 803, and resistor 805, along with a secondpulse width modulator 807.

An input current monitoring circuit may be configured to generate asignal that is representative of an average input current to the primarywinding. As illustrated in FIG. 8, the input current monitoring circuitmay include the sense resistor 319, the peak input current sensingcircuit 321, the second pulse width modulator 807, and a low pass filterformed by the resistor 805 and the capacitor 803. In this case, thesecond pulse width modulator 807 may multiply the peak input currentthat is sensed by the peak input current sensing circuit 321 by a dutycycle signal 815 that is representative of the duty cycle of current inthe primary winding 303. The duty cycle signal 815 may be derived fromthe Q output of the D memory 331. This pulse-width modulated signal maybe filtered by the low pass filter formed by the resistor 805 and thecapacitor 803, thus generating an average input current signal 811 atthe minus input to the amplifier 801. The low pass filter may beconfigured to have a cut-off frequency that is between the frequency ofthe switching signal to the FET 317 and the frequency of the chopped andrectified AC voltage. For example, when the switching signal is atapproximately 200 KHz and the chopped and rectified AC voltage is atapproximately 120 hertz, the cut-off frequency of the low pass filtermay be approximately, 10 KHz.

This configuration may alter the nature of what the output from themultiplier 501 represents. In FIG. 8, the output from the multiplier 501may now represent a desired average input current signal 815. Theamplifier 801, the capacitor 803, and the resistor 805 may form a secondintegrator which integrates the difference between the desired averageinput current 815 and the average input current signal 811, thusgenerating the desired peak input current signal 339.

By causing the desired average input current signal to track the inputvoltage, rather than the desired peak input current signal, the powerfactor may be increased to at least 0.99 for all settings of the dimmercontrol 105.

The circuits illustrated in FIGS. 1, 3, 5, and 8 may generate a ripplein the output current that is delivered to the LEDs. The amount of thisripple may depend upon the amount of output capacitance which is used inthe output filter 123, such as in the capacitor 309, as well as theamount of voltage and current that are required by the LEDs.

The ripple may have two components. The first component may be due tothe switching signal from the flyback controller. However, this may bevery high in frequency, such as at about 200 KHz, and thus easilyfiltered by small values in output capacitance.

The second component may be due to the chopped and rectified AC voltage.This second component may be much lower in frequency, such as at about120 hertz, and may require extremely large values of capacitance tofilter. For example, a 10 watt set of LEDs that are operated at 50 voltsmay require a capacitance in excess of 10,000 microfarads to adequatelyfilter the 120 hertz ripple. Such a capacitance can be expensive, bulky,and prone to failure.

FIG. 9 illustrates a current ripple reduction circuit. The circuitillustrated in FIG. 9 may be used in conjunction with the circuitsillustrated in FIGS. 1, 3, 5, and 8, as well as in connection with othertypes of LED circuits. Similarly, the circuits illustrated in FIGS. 1,3, 5, and 8 may be used in connection with other types of current ripplereduction circuits.

The current ripple reduction circuit may be connected to a power supply.The power supply may include a rectifying diode, such as a diode 906.

The current ripple reduction circuit may be connected to one or moreLEDs that are connected in series, in parallel, or in series andparallel. For example, and as illustrated in FIG. 9, LEDs 901, 903, and905 may be connected in series. The LEDs 901, 903, and 905 may be any ofthe types of LEDs discussed above, and a different number may be usedinstead.

The current ripple reduction circuit may include a capacitance, such asa capacitor 904. The capacitor 904 may be configured to filter outputfrom a secondary winding of a transformer in a flyback converter afterit is rectified by a diode, such as the diode 906. The value of thecapacitance may be selected so as to filter high frequency currentripple caused by a switching signal in the flyback converter, but toonly partially filter current ripple caused by the chopping of a lowfrequency chopped and rectified AC voltage source, such as by a dimmercontrol. For example, a value in the range of 1 to 1000 microfarads orfrom 2 to 20 microfarads may be used. The value of the capacitor 904 maybe such as to allow the ripple in the output voltage across thiscapacitance that is attributable to the chopped and rectified AC voltageto be as much as 10% of the peak value of the output voltage.

The current ripple reduction circuit may include a current regulator,such as a current regulator 902, that is connected in series with theLEDs. The current regulator 902 may be configured to substantiallyreduce fluctuations in the current which flows through the LEDs due tothe low frequency ripple component of the output current, but notfluctuations in the current which flows through the LEDs due to changesin an average value of the output current.

The current regulator 902 may include a controllable, constant currentsource, such as a FET 908. The FET 908 may be configured to conduct aconstant amount of current from a source 907 through a drain 909 that isapproximately proportional to an input voltage at a gate 911. The inputvoltage to the gate 911 may be developed from a low pass filter that mayinclude a resistance and a capacitance, such as a resistor 913 and acapacitor 915, respectively.

The low pass filter may be configured to deliver a voltage to the gate911 of the FET 908 that is substantially proportional to the averagevalue of the output current with the low frequency ripple componentbeing substantially attenuated. In order to accomplish this, the lowpass filter may be configured to have a cut-off frequency that is atleast five times less than the low frequency ripple of the chopped andrectified AC voltage, such as approximately ten times less.

Although the LEDs 901, 903, and 905 are illustrated as being in serieswith the source of the FET 908, they may be instead be in series withthe drain 909 of the FET 908. Also, other types of current regulatorsmay be used, instead of the one illustrated in FIG. 9.

FIG. 10 illustrates part of a flyback controller that may be used in aflyback converter driven by a dimmer control to enhance the perceivedlinearity between changes in the settings of the dimmer control andcorresponding changes in the intensity of light from one or more LEDsdriven by the flyback converter. The circuitry illustrated in FIG. 10may be used in connection with the circuits illustrated in FIGS. 3, 5,and 8, by replacing the amplifier 335 with an amplifier 1001 and byadding the additional components that are illustrated in FIG. 10 and arenow described.

As illustrated in FIG. 105 a tracking input 1003 may be configured toreceive a dimmer output tracking signal that is representative of theinstantaneous magnitude of the output from a dimmer control. The dimmeroutput tracking signal may, for example, be a scaled version of thechopped and rectified AC voltage that is delivered by the output of therectification system 111 illustrated in FIG. 1. The rectification system111 may, for example, be a full wave bridge rectifier.

An averaging circuit may be configured to average the dimmer outputtracking signal at the tracking input 1003 so as to generate an averagedimmer output signal 1005 that is representative of an average of thedimmer output tracking signal. The averaging circuit may include a lowpass filter which may include a resistor 1007, a resistor 1009, and acapacitor 1011. The low pass filter may be configured to have a cut-offfrequency that is at least five times less than the frequency of thedimmer output tracking signal, such as approximately 10 times less thanthis frequency. For example, the dimmer output tracking signal may havea frequency of about 120 hertz, in which event the low pass filter mayhave a cut-off frequency of about 12 hertz.

The amplifier 1001 may be configured with the resistor 325 and thecapacitor 327 so as to function as integrator. The amplifier 1001 mayinclude a least value circuit 1013 configured to output the lesser ofthe desired average output current signal 337 and the average dimmeroutput signal 1005. The amplifier 1001 may be configured to integratethe difference between the output of the least value circuit 1013 andthe average output current signal 333.

The net effect of this circuitry modification may be to substitute theaverage dimmer output signal 1005 for the desired average output currentsignal 337 at such times as the average dimmer output signal 1005 isless than the desired average output current signal 337. This may helpensure that the flyback converter does not try and maintain the outputcurrent at a high level after a setting on the dimmer control has beenadjusted to call for a lower current output.

The desired average output current signal 337 may function as athreshold in connection with the phase angle of the chopped AC voltagefrom the dimmer control 105. For example, the desired average outputcurrent signal 337 may be set to exceed the average dimmer signal 1005at a 0 degree phase angle. This may cause the average dimmer signal 1005to control the average current output of the flyback converterthroughout all of the various phase angle settings of the dimmercontrol.

The desired average output current signal 337 may instead be set toequal the average dimmer signal 1005 at a phase angle that is between 0and 180 degrees, such as at about 90 degrees. With this setting, thedesired average output current signal 337 may control the desiredaverage output current for all phases angles that are less than 90degrees, while the average dimmer signal 1005 may control the desiredaverage output current at all larger phase angles. The desired averageoutput current signal 337 may instead be set to equal the average dimmersignal 1005 at other phase angles, such as at 45 degrees.

FIG. 11 is a graph of output current as a function of dimmer controlsettings for various flyback converter designs. A flyback converterdesign that lacks the circuitry illustrated in FIG. 10 may have a linearrelationship between its output current and the phase angle of thedimmer control setting, as illustrated by a straight line 1101 in FIG.11. If the desired average output current signal 337 is set to exceedthe average dimmer signal 1005 at a 0 degree phase angle, a scallopedcurve 1103 may be illustrative of the relationship between the settingof the dimmer and the current output of the flyback converter. Ifinstead the desired average output current signal 337 is set to equalthe average dimmer control signal 1005 at a phase angle of about 90degrees, then the bifurcated curve 1105 may illustrate the relationshipbetween the setting of the dimmer control and the output current.

Using such a “cross-over” setting may provide greater immunity to noisein the line voltage during low phase angle settings of the dimmercontrol. Setting the cross-over point at about 90 degrees may also causethe intensity of light from the LEDs to appear to a human eye to trackchanges in the setting of the dimmer control for phase angles largerthan 90 degrees in a fashion that varies more linearly with the settingof the dimmer control. This may occur because of the non-linear mannerin which the human brain interprets changes in luminance levels.

As indicated in the foregoing Description of Related Art, a dimmercontrol may leak current while its triac is not firing. This may causethe voltage in the flyback converter to rise during off periods of thechopped and rectified AC voltage. In turn, this may create noise,flickering, and/or other problems or concerns.

FIG. 12 illustrates a flyback controller configured to prevent voltagebuildup in a flyback converter that is being driven by a dimmer controldue to leakage in the dimmer control. The features that are illustratedin FIG. 12 and that will now be discussed may be used in connection withthe flyback controllers or portions thereof which are illustrated inFIGS. 1, 3, 5, 8, and 10, or in any other type of flyback controller.Similarly, the flyback controllers or portions thereof which areillustrated in FIGS. 1, 3, 5, 8, and 10 may be used in connection withother types of circuitry to prevent voltage buildup due to leakage inthe dimmer control.

As illustrated in FIG. 12, a flyback controller 1201 may be configuredto generate a switching signal 1203 that may be delivered to a switchingsystem, such as was described above in connection with FIGS. 1, 3, 5and/or 8. The flyback controller may have a switching signal generatorcircuit 1204 that may be configured to generate the switching signal1203 to conform to any desired flyback controller switching signaltiming, such as one of the timings discussed above in connection withFIGS. 1-10. The switching signal generator circuit 1204 may include anytype of circuit, such as one of the types of circuits discussed above inconnection with FIGS. 1-10.

The flyback controller 1201 may have a control circuit 1205. The controlcircuit may have a comparator 1207, a threshold value generator circuit1209, and an OR gate 1211. The threshold value generator circuit 1209may be configured to generate a threshold value above which a signalrepresentative of the chopped and rectified AC voltage may be consideredto be in an on period, and below which the signal that is representativeof the chopped and rectified AC voltage may be considered to be in anoff period. For example, the threshold may be set at less than 10% ofthe peak value of the signal which is representative of the chopped andrectified AC voltage, at less than 5% of this peak value, or at someother value.

The comparator 1207 may be configured to compare the instantaneous valueof the signal that is representative of the chopped and rectified ACvoltage with the threshold generated by the threshold value generatorcircuit 1209. During such time as the signal that is representative ofthe chopped and rectified AC voltage is higher than the threshold, nosignal may be delivered to the OR gate 1211, causing the switchingsignal 1203 to be governed by the output from the switching signalgenerator circuit 1204. During such times as the signal that isrepresentative of the chopped and rectified AC voltage is less than thethreshold, however, the comparator 1207 may generate a positive output,causing the switching signal 1203 to be in its on state, regardless ofthe signal from the switching signal generator circuit 1204.

FIG. 13 illustrates wave forms that may be present in the flybackcontroller illustrated in FIG. 12. As illustrated in FIG. 13, theswitching signal 1203 may remain high during a period 1303 when thechopped and rectified AC voltage 1301 is off. When the chopped andrectified AC voltage 1301 is firing during a period 1305, on the otherhand, the switching signal 1203 may oscillate as it normally does so asto cause the average output current in the secondary winding of theflyback controller to be at a desired level.

As also illustrated in FIG. 13, the switching signal 1203 may remainhigh at the commencement of the period 1305, thereby beginning the firstoscillation of the switching signal after the chopped and rectified ACvoltage switches from an off period to an on period.

The net effect of the circuit illustrated in FIG. 12 may be to load thedimmer control with the primary winding of the transformer at such timesas the dimmer control is not firing. This may bleed any leakage currentand thus prevent a voltage buildup during such off periods, withoutrequiring any additional active high voltage device or devices. Othercircuitry techniques for effectuating the same type of signal control ofthe switching system may be used in addition or instead.

The various components which have been described may be packaged in anyway. For example, the components that comprise the flyback controllermay be packaged in a single integrated circuit with other active andpassive components, a set of integrated circuits with other active andpassive components, or a set of discrete transistor circuits with otheractive and passive components.

All of the various circuits that have been described may be used inconnection with one another in any and all combinations.

The components, steps, features, objects, benefits and advantages thathave been discussed are merely illustrative. None of them, nor thediscussions relating to them, are intended to limit the scope ofprotection in any way. Numerous other embodiments are also contemplated,including embodiments that have fewer, additional, and/or differentcomponents, steps, features, objects, benefits and advantages. Thecomponents and steps may also be arranged and ordered differently.

The phrase “means for” when used in a claim embraces the correspondingstructures and materials that have been described and their equivalents.Similarly, the phrase “step for” when used in a claim embraces thecorresponding acts that have been described and their equivalents. Theabsence of these phrases means that the claim is not limited to any ofthe corresponding structures, materials, or acts or to theirequivalents.

Nothing that has been stated or illustrated is intended to cause adedication of any component, step, feature, object, benefit, advantage,or equivalent to the public, regardless of whether it is recited in theclaims.

In short, the scope of protection is limited solely by the claims thatnow follow. That scope is intended to be as broad as is reasonablyconsistent with the language that is used in the claims and to encompassall structural and functional equivalents.

1. A flyback controller for generating a switching signal that causes aswitching system in a flyback converter to deliver current to a primarywinding of a transformer in the flyback converter while the switchingsignal is in an on state and to stop delivering that current while theswitching signal is in an off state, the flyback controller comprising:a dimmer input configured to receive a dimmer signal reflective of achopped and rectified AC voltage, each cycle of which has an off periodwhich is substantially attenuated but not always zero due to leakage ofa dimmer control from which the chopped AC voltage originates and an onperiod which substantially tracks the AC voltage, the ratio of the offperiod to the on period being dependent upon a setting of the dimmercontrol; and a control circuit configured to generate the switchingsignal based on the dimmer signal from the dimmer input such that theswitching signal: controllably oscillates between its on and off statesduring the on periods of the chopped and rectified AC voltage so as tocontrollably regulate the current that is delivered by a secondarywinding of the transformer; and is in the on state during the offperiods of the chopped and rectified AC voltage, thereby preventing avoltage build up from the dimmer control leakage.
 2. The flybackcontroller of claim 1 wherein the control circuit includes a comparatorconfigured to compare the dimmer signal from the dimmer input with athreshold value and wherein the control circuit is configured toindicate that the chopped and rectified AC voltage is in an on periodwhen the comparator indicates that the chopped and rectified AC voltageis greater than the threshold and is in an off period when thecomparator indicates that the chopped and rectified AC voltage is lessthan the threshold.
 3. The flyback controller of claim 2 wherein the ACvoltage has a maximum and wherein the threshold is less than ten percent(10%) of the maximum.
 4. The flyback controller of claim 3 wherein thethreshold is less than five percent (5%) of the maximum.
 5. The flybackcontroller of claim 1 wherein the control circuit is configured to causethe switching signal to oscillate during the on periods of the choppedand rectified AC voltage in a manner that causes the secondary windingof the transformer to deliver an average current that tracks the averagevalue of the chopped and rectified AC voltage in a linear or non-linearfashion.
 6. The flyback controller of claim 5 wherein the controlcircuit is configured to cause the tracking to be in a substantiallylinear fashion.
 7. The flyback controller of claim 5 wherein the controlcircuit is configured to provide power factor correction.
 8. The flybackcontroller of claim 1 wherein the control circuit is configured to causethe switching signal to remain in the on state after the chopped andrectified AC voltage switches from the off period to the on period,thereby beginning the first cycle of the switching signal.
 9. A poweredLED circuit comprising: a flyback converter comprising: a rectificationsystem configured to produce a chopped and rectified AC voltage byrectifying a chopped AC voltage from a dimmer control, each cycle ofwhich has an off period which is substantially attenuated but not zerodue to leakage of the dimmer control and an on period whichsubstantially tracks the AC voltage, the ratio of the off period to theon period being dependent upon a setting of the dimmer control; atransformer having a primary winding and a secondary winding; aswitching system configured to controllably deliver current into theprimary winding of the transformer; and a flyback controller configuredto generate a switching signal that causes the switching system todeliver current to the primary winding of the transformer while theswitching signal is in an on state and to stop delivering that currentwhile the switching signal is in an off state, the flyback controllercomprising a control circuit configured to generate the switching signalbased on the chopped and rectified AC voltage such that the switchingsignal: controllably oscillates between its on and off states during theon periods of the chopped and rectified AC voltage so as to controllablyregulate the current that is delivered by the secondary winding of thetransformer; and is in the on state during the off periods of thechopped and rectified AC voltage, thereby preventing a voltage build upfrom the dimmer control leakage; and one or more LEDs configured toreceive current that is delivered by the secondary winding of thetransformer.
 10. The powered LED circuit of claim 9 wherein the controlcircuit includes a comparator configured to compare the dimmer signalfrom the dimmer input with a threshold value and wherein the controlcircuit is configured to indicate that the chopped and rectified ACvoltage is in an on period when the the comparator indicates thatchopped and rectified AC voltage is greater than the threshold and is inan off period when the comparator indicates that the chopped andrectified AC voltage is less than the threshold.
 11. The powered LEDcircuit of claim 10 wherein the AC voltage has a maximum and wherein thethreshold is less than ten percent (10%) of the maximum.
 12. The poweredLED circuit of claim 11 wherein the threshold is less than five percent(5%) of the maximum.
 13. The powered LED circuit of claim 9 wherein thecontrol circuit is configured to cause the switching signal to oscillateduring the on periods of the chopped and rectified AC voltage in amanner that causes the secondary winding of the transformer to deliveran average current that tracks in a linear or non-linear fashion theaverage value of the chopped and rectified AC voltage.
 14. The poweredLED circuit of claim 13 wherein the control circuit is configured tocause the tracking to be in a substantially linear fashion.
 15. Thepowered LED circuit of claim 13 wherein the control circuit isconfigured to provide power factor correction.
 16. The powered LEDcircuit of claim 9 wherein the rectification system has an output thatis filtered by capacitance in an amount that is insufficient tosubstantially alter the fundamental wave shape of the chopped andrectified AC voltage.
 17. The powered LED circuit of claim 16 whereinthe capacitance is no more than 1 micro farad.
 18. The powered LEDcircuit of claim 9 further comprising a dimmer control configured toproduce the chopped AC voltage.
 19. The powered LED circuit of claim 18in which the dimmer control includes a triac configured to product thechopped AC voltage.
 20. The powered LED circuit of claim 9 wherein thecontrol circuit is configured to cause the switching signal to remain inthe on state thereby beginning the first cycle of the switching signalafter the chopped and rectified AC voltage switches from the off periodto the on period.
 21. A flyback controller for generating a switchingsignal that causes a switching system in a flyback converter to delivercurrent to a primary winding of a transformer in the flyback converterwhile the switching signal is in an on state and to stop delivering thatcurrent while the switching signal is in off state, the flybackcontroller comprising: means for receiving a dimmer signal reflective ofa chopped and rectified AC voltage, each cycle of which has an offperiod which is substantially attenuated but not zero due to leakage ofa dimmer control from which the chopped AC voltage originates and an onperiod which substantially tracks the AC voltage, the ratio of the offperiod to the on period being dependent upon a setting of the dimmercontrol; and means for generating the switching signal based on thedimmer signal from the means for receiving such that the switchingsignal: controllably oscillates between its on and off states during theon periods of the chopped and rectified AC voltage so as to controllablyregulate the current that is delivered by a secondary winding of thetransformer; and is in the on state during the off periods of thechopped and rectified AC voltage thereby preventing a voltage build upfrom the leakage.
 22. A powered LED circuit comprising: a flybackconverter comprising: means for producing a chopped and rectified ACvoltage from a chopped AC voltage from a dimmer control, each cycle ofwhich has an off period which is substantially attenuated but not zerodue to leakage of the dimmer control and an on period whichsubstantially tracks the AC voltage, the ratio of the off period to theon period being dependent upon a setting of the dimmer control; atransformer having a primary winding and a secondary winding; means forcontrollably delivering current into the primary winding of thetransformer; and means for generating a switching signal that causes themeans for controllably delivering current to deliver current to theprimary winding of the transformer while the switching signal is in anon state and to stop delivering that current during while the switchingsignal is in off state, the means for generating causing the switchingsignal to be based on the chopped and rectified AC voltage such that theswitching signal: controllably oscillates between its on and off statesduring the on periods of the chopped and rectified AC voltage so as tocontrollably regulate the current that is delivered by the secondarywinding of the transformer; and is in the on state during the offperiods of the chopped and rectified AC voltage, thereby preventing avoltage build up from the leakage; and one or more LEDs configured toreceive current that is delivered by the secondary winding of thetransformer.